One person is dead and dozens are injured after a Singapore Airlines flight from London struck "severe turbulence" and was forced to make an emergency landing in Bangkok.

The Boeing 777-300ER plane with 211 passengers and 18 crew was headed to Singapore when it diverted at 3:45pm (local time) on Tuesday, the airline said in a statement.

The flight fell into an air pocket while the cabin crew was serving breakfast, Suvarnabhumi airport general manager Kittipong Kittikachorn told a press conference.

He said a 73-year-old British man died during the incident, likely due to a heart attack.

Singapore Airlines also confirmed the death and said 18 people were hospitalised, including a crew member.

Mr Kittikachorn said head injuries were sustained among seven people who were critically injured.

At least 12 people are being treated in hospitals, the airlines said, with the remaining passengers and crew receiving treatment, where necessary, at the airport.

"Singapore Airlines offers its deepest condolences to the family of the deceased," the airlines said in a statement.

"Our priority is to provide all possible assistance to all passengers and crew on board the aircraft."

Dzafran Azmir, a 28-year-old student on board the flight, told Reuters the aircraft started "tilting up and there was shaking". 

"So I started bracing for what was happening, and very suddenly there was a very dramatic drop," he said.

"Some people hit their heads on the baggage cabins overhead and dented it, they hit the places where lights and masks are and broke straight through it."

Uninjured passengers have disembarked and an another aircraft will fly them onwards, Suvarnabhumi airport said.

Singapore's Minister for Transport Chee Hong Tat said in a statement he was "deeply saddened to learn about the incident" on flight SQ231. 

"Ministry of transport, Singapore, Singapore ministry of foreign affairs, civil aviation authority of Singapore and Changi airport officials, as well as SIA staff, are providing support to the affected passengers and their families," he said.

"My deepest condolences to the family of the deceased."

Boeing said it was in contact with Singapore Airlines and was ready to provide support.

The last Singapore Airlines fatalities were in October 2000 when a plane crashed on a closed runway during take-off in Taiwan. Eighty-three people died.

Singapore Airlines has had seven accidents, according to records by the Aviation Safety Network.

ABC/Reuters